I’ve been using a Sony Epaper notebook for almost 2 years now and considering buying a Boox Note 5 or Air 2 (the remarkable didn’t make my short list)
They are running almost stock android and the quality seems better than remarkable.
I’ve watched My Deep Guide (https://youtube.com/c/MyDeepGuide) where they test every single feature of all note taking devices.
This article says that you have root access on the remarkable, but doing so will void the warranty.
rM support's statements in the past have been to the effect of "we aren't going to remove the root access; you need it for GPL compliance, and we need it for troubleshooting. And merely using it doesn't void the warranty, but we don't support third-party modifications." So if there's e.g. a flaw in the hardware that you didn't obviously cause by doing something absurd with modifications (e.g. overriding the temperature lockouts, deliberately wearing out the eMMC with writes, ...) they say that it shouldn't really effect your claim.
Now individual experiences with rM support have varied rather wildly between "they're great!" and "they're the worst!" so feel free to temper your expectations.
If you've been using a Sony DPT-RP1 or DPT-CP1, you should also consider the Fujitsu Quaderno Gen 2, which is the most prominent successor of the RP1/CP1. I strongly prefer my Quaderno to my Remarkable 2. Unfortunately My Deep Guide hasn't been able to get a review unit yet.
Damn! I checked the reviews of the Quaderno Gen 2: I am strongly considering this one!
The build quality of the RP1/CP1 is so good (the texture of both the screen and the back is a delight), i hope it feels the same on the Fujitsu.
The chassis is virtually the same, apart from the addition of USB-C and a relocating of the wrist strap holes to the top. I actually use the official Sony CP1 case on my A5 Quaderno Gen 2 just because I happened to find that case on clearance. The main improvements are speed-related, screen related (noticeably better contrast) and most of all, a dramatic improvement in pen feel, speed and accuracy. Also some nice functionality improvements, especially around the highlighting function. There's also now a freehand highlighter.
Have a look at the reinkstone, that looks quite interesting. I have a remarkable and a sony. I haven't used the sony since buying the remarkable. Despite the larger size, it is a much worse device. The writing experience really doesn't compare.
An alternative if you want more functionality is to root your sony, there was some progress on hacking the sony for putting regular android packages on it. I can dig out some links if you can't find it.
You will need to root it. It’s running a very custom android. There is a good documentation about it. Look for DPTTools.
But if you want a epaper tablet with the play store (and certified) look at the BOOX ones (note, air or Lumi). You can install the play store and download the official kindle app.
Some says that BOOX has questionable GPL practices and does some api call in China (might be just telemetry), check in this forum for opinions:).
This article says that you have root access on the remarkable, but doing so will void the warranty.